1. Technical Field
The present disclosure is directed to a temperature switch with a resistive sensor, and more particularly, a sensor that includes a Wheatstone bridge having three resistors each having a low temperature coefficient of resistance and a fourth resistor with a high temperature coefficient of resistance.
2. Description of the Related Art
Many electronic devices have temperature ranges within which they operate. If the device is in an environment that is outside of the temperature range, the reliability of the device is affected, such that the device may cease to operate correctly or at all.
Smart phones, such as iPhones, Blackberries, and other cellular telephones or mobile devices, are examples of devices that have a temperature range for optimum performance. There are recommended ranges for operating temperatures and storage temperatures. The recommended operating temperatures are in the range of 0 and 75 degrees Celsius. The recommended storage temperatures are in the range of −20 and 45 degrees Celsius. If the smart phone is in an environment that is outside of these ranges or the phone itself heats up to a temperature outside of these ranges, the phone may stop charging, dim the display, experience a decrease in cellular signal, disable location services, or disable the camera flash. In some phones a temperature warning screen will appear and indicate that the phone needs to cool down before further use.
In order to prevent damage to the phone or other electronic device from heat, temperature sensors are integrated into the phones and devices. For example, in the smart phones, a temperature switch can trigger a safe mode when any of the extreme borders of optimal temperature ranges are reached. The safe mode can shut the phone off or limit power consumption to decrease the operating temperature of the phone.
A variety of temperature switches are used to turn off the phone or device or turn off a component of the phone or device if one of the border temperatures is detected. For example, a thermocouple may be included on a printed circuit board of the phone or device to detect a change in temperature. The thermocouple is configured to produce a voltage proportional to a temperature difference between two conductors. One of the limitations of the thermocouple is accuracy and precision.
FIGS. 1 and 2 are existing solutions of circuitry used for temperature switches where a threshold temperature value is an input into circuitry separate from a sensor that detects the temperature. FIG. 1 is a resistor-programmable temperature switch 100 made by Texas Instruments and referred to as TMP300. The temperature switch 100 uses an external set resistor RSET to program a trip temperature for the output, OUT. The temperature is sensed in a sensor separate from the temperature switch 100 and provided to the temperature switch 100 as an electrical signal, VTEMP. The signal VTEMP is compared to the set resistor value representing a threshold temperature TSET. In this switch, the threshold temperature values are input into the electronics and not into the sensor.
FIG. 2 is user programmable temperature switch 200 that can be programmed with a single external resistor RSET, which is made by Microchip and referred to as MCP9510. This switch 200 is capable of triggering for temperatures between −40 and 125 degrees Celsius. The user selects a resistance of RSET, to program a threshold temperature at which to trigger the temperature switch 200. A voltage VSET across RSET is compared to an internal thermal diode 202 that is configured to sense the temperature. A voltage of the thermal diode 202 is compared to the voltage VSET across RSET with a comparator 204. If the voltage of the thermal diode 202 exceeds VSET then an output OUT will be activated.
As with the temperature switch 100 in FIG. 1, the temperature switch 200 in FIG. 2 requires the temperature threshold value to be input into electronics adjacent to the temperature switch and is not provided integrally with the temperature switch. Both of these temperature switches 100, 200 are separate components from the other components of a telephone or other electronic device, such as a processor, a mother board, and a crystal oscillator. The temperature switches 100, 200 plus additional external resistors must be attached to the printed circuit board onto which the processor and other components are attached.